<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <title>Document</title>
</head>

<body>
  <div id="app">
    <h1>counter: {{counter}}</h1>
    <button @click='add'>+</button>
    <button @click='sub'>-</button>
  </div>

  <script src="../js/vue.js"></script>
  <script>
    const app = new Vue({
      el: '#app',
      data: {
        counter: 0
      },
      methods: {
        add() {
          this.counter++;
        },
        sub() {
          this.counter--;
        }
      }
    })
  </script>

</body>

</html>